[K].[S:2]([C:6]1[C:7]([C:11]([OH:13])=O)=[CH:8][S:9][CH:10]=1)(O)(=[O:4])=[O:3].P(Cl)(Cl)(Cl)(Cl)[Cl:15].[ClH:20]>O=P(Cl)(Cl)Cl>[Cl:20][S:2]([C:6]1[C:7]([C:11]([Cl:15])=[O:13])=[CH:8][S:9][CH:10]=1)(=[O:4])=[O:3]94.6 g (0.384 mole) of the monopotassium salt of 4-sulfothiophene-3-carboxylic acid (V) is suspended in 390 ml of POCl3 ; while stirring, 160.8 g (0.768 mole) of PCl5 is added (accompanied by vigorous evolution of HCl). The mixture is then heated for 3 hours on a water bath while stirring and then cooled to room temperature, inorganic salts are filtered off and the POCl3 is distilled off, as far as possible, at subatmospheric pressure. The residue is dissolved in 400 ml of dry chloroform to remove any inorganic salts which may still be present, filtered and evaporated. The residue crystallizes upon cooling and may be used in the next stage without further purification; melting point: 78° to 79° C.; yield: 90.8 g (96.5%).
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